Determining microcode entry points and prefix bytes using a parallel logic technique

ABSTRACT

A superscalar microprocessor implements a microcode instruction unit with an MROM entry point generator. The MROM entry point generator generates the first address of a microcode sequence that effectuates the function of an MROM instruction. The address of the microcode instruction depends upon the prefix bytes, the opcode bytes and the ModR/M byte of the MROM instruction. To expedite the generation of the first address, or entry point address, a plurality of entry point address generators generate entry point addresses based on a presumed state of the prefix byte. In parallel with the generation of the entry point addresses, a prefix decoder decodes the prefix byte to determine the state of the prefix byte. The output of the entry point address generator that generated an entry point address based on the actual prefix byte is selected. By decoding the prefix byte in parallel with entry point address generation, the propagation delay associated generation of entry point addresses is advantageously reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of microprocessors and, more particularly, to microcode instruction mechanisms within microprocessors.

2. Description of the Relevant Art

Superscalar microprocessors achieve high performance by executing multiple instructions per clock cycle and by choosing the shortest possible clock cycle consistent with the design. As used herein, the term "clock cycle" refers to an interval of time accorded to various stages of an instruction processing pipeline within the microprocessor. Storage devices (e.g. registers and arrays) capture their values according to the clock cycle. For example, a storage device may capture a value according to a rising or falling edge of a clock signal defining the clock cycle. The storage device then stores the value until the subsequent rising or falling edge of the clock signal, respectively. The term "instruction processing pipeline" is used herein to refer to the logic circuits employed to process instructions in a pipelined fashion. Although the pipeline may be divided into any number of stages at which portions of instruction processing are performed, instruction processing generally comprises fetching the instruction, decoding the instruction, executing the instruction, and storing the execution results in the destination identified by the instruction.

Microprocessor designers often design their products in accordance with the x86 microprocessor architecture in order to take advantage of its widespread acceptance in the computer industry. Because the x86 microprocessor architecture is pervasive, many computer programs are written in accordance with the architecture. X86 compatible microprocessors may execute these computer programs, thereby becoming more attractive to computer system designers who desire x86-capable computer systems. Such computer systems are often well received within the industry due to the wide range of available computer programs.

The x86 instruction set is relatively complex and is characterized by a plurality of variable byte-length instructions. A generic format illustrative of the x86 instruction set is shown in FIG. 1A. As illustrated in the figure, an x86 instruction consists of from one to four optional prefix bytes 102, followed by an operation code (opcode) field 104, an optional addressing mode (Mod R/M) byte 106, an optional scale-index-base (SIB) byte 108, an optional displacement field 110, and an optional immediate data field 112.

The opcode field 104 defines the basic operation for a particular instruction. The default operation of a particular opcode may be modified by one or more prefix bytes. For example, a prefix byte may be used to change the address or operand size for an instruction, to override the default segment used in memory addressing, to instruct the processor to repeat a string operation a number of times, or to specify a different basic operation. The prefix bytes may contain one or more prefix byte codes. The opcode field 104 follows the prefix bytes 102, if any, and may be one or two bytes in length. The addressing mode (ModR/M) byte 106 specifies the registers used as well as memory addressing modes. The scale-index-base (SIB) byte 108 is used only in 32-bit base-relative addressing using scale and index factors. A base field of the SIB byte specifies which register contains the base value for the address calculation, and an index field specifies which register contains the index value. A scale field specifies the power of two by which the index value will be multiplied before being added, along with any displacement, to the base value. The next instruction field is the optional displacement field 110, which may be from one to four bytes in length. The displacement field 110 contains a constant used in address calculations. The optional immediate field 112, which may also be from one to four bytes in length, contains a constant used as an instruction operand. The 80286 sets a maximum length for an instruction at 10 bytes, while the 80386 and 80486 both allow instruction lengths of up to 15 bytes.

Referring now to FIG. 1B, several different variable byte-length x86 instruction formats are shown. The shortest x86 instruction is only one byte long, and comprises a single opcode byte as shown in format (a). For certain instructions, the byte containing the opcode field also contains a register field as shown in formats (b), (c) and (e). Format (j) shows an instruction with two opcode bytes. An optional ModR/M byte follows opcode bytes in formats (d), (f), (h), and (j). Immediate data follows opcode bytes in formats (e), (g), (i), and (k), and follows a ModR/M byte in formats (f) and (h). FIG. 1C illustrates several possible addressing mode formats (a)-(h). Formats (c), (d), (e), (g), and (h) contain ModR/M bytes with offset (i.e., displacement) information. An SIB byte is used in formats (f), (g), and (h).

Certain instructions within the x86 instruction set are quite complex, specifying multiple operations to be performed. For example, the PUSHA instruction specifies that each of the x86 registers be pushed onto a stack defined by the value in the ESP register. The corresponding operations are a store operation for each register, and decrements of the ESP register between each store operation to generate the address for the next store operation. Often, complex instructions are classified as MROM instructions. MROM instructions are transmitted to a microcode unit within the microprocessor, which decodes the complex MROM instruction and produces two or more simpler microcode instructions for execution by the microprocessor. The simpler microcode instructions corresponding to the MROM instruction are typically stored in a read-only memory (ROM) within the microcode unit. The microcode unit determines an address within the ROM at which the simpler microcode instructions are stored, and transfers the microcode instructions out of the ROM beginning at that address. Multiple clock cycles may be used to transfer the entire set of microcode instructions corresponding to the MROM instruction. The entire set of microcode instructions that effectuate the function of an MROM instruction is called a microcode sequence. Each MROM instruction may correspond to a particular number of microcode instructions dissimilar from the number of microcode instructions corresponding to other MROM instructions. Additionally, the number of microcode instructions corresponding to a particular MROM instruction may vary according to the addressing mode of the instruction, the operand values, and/or the options included with the instruction. The microcode unit issues the microcode instructions into the instruction processing pipeline of the microprocessor. The microcode instructions are thereafter executed in a similar fashion to other instructions. It is noted that the microcode instructions may be instructions defined within the instruction set, or may be custom instructions defined for the particular microprocessor.

Conversely, less complex instructions are decoded by hardware decode units within the microprocessor, without intervention by the microcode unit. The terms "directly-decoded instruction" and "fastpath instruction" will be used herein to refer to instructions which are decoded and executed by the microprocessor without the aid of a microcode unit. As opposed to microcode instructions which are reduced to simpler instructions which may be handled by the microprocessor, directly-decoded instructions are decoded and executed via hardware decode and functional units included within the microprocessor.

Unfortunately, decoding MROM instructions is a relatively complex and time consuming procedure. Because the prefix bytes may modify the basic operation of an MROM instruction, the prefix bytes are decoded prior to decoding the opcode and ModR/M bytes. In high frequency microprocessors, the sequential steps of decoding the prefix bytes and subsequently decoding the opcode and ModR/M bytes may exceed the period of a clock cycle. This delay can create stalls in instruction dispatch and thereby reduce the throughput of the microprocessor.

SUMMARY OF THE INVENTION

The problems outlined above are in large part solved by an microcode instruction unit in accordance with the present invention. The microcode instruction unit, or MROM instruction unit, includes an entry point circuit that generates a first address of a microcode sequence that effectuates the function of the MROM instruction. The entry point circuit includes a plurality of address generators. Each address generator generates an entry point address based on one of the possible prefix bytes. There is one address generator for each of the possible prefix byte combinations that effects the starting address of the microcode sequence. In parallel with address generation, a prefix decoder decodes the prefix byte. A multiplexor selects the output of the decoder which corresponds to the actual prefix byte of the MROM instruction. The present invention advantageously accommodates expedient decoding of MROM instructions. The decoding of the prefix bytes is performed in parallel with address generation.

Broadly speaking, the present invention contemplates a microcode instruction unit including a storage device, a plurality of address generators, and a selection circuit. The storage device is configured to store a plurality of microcode instructions, wherein a subset of the plurality of microcode instructions effectuates a function of a complex instruction, and the complex instruction includes an opcode byte and a prefix byte. The plurality of address generators is configured to generate an address of a first microcode instruction of the plurality of microcode instructions that effectuate the function of the complex instruction, wherein a first address generator generates a first address based on the opcode byte and a first presumed prefix byte, and a second address generator generates a second address based on the opcode byte and a second presumed prefix byte. The selection circuit coupled to the plurality of address generators and to the storage device, wherein the selection circuit is configured to select the first address if the first presumed prefix byte corresponds to the prefix byte of the complex instruction, and the selection circuit is configured to select the second address if the second presumed prefix byte corresponds to the prefix byte of the complex instruction.

The invention further contemplates a method of accessing a microcode sequence that effectuates a function of a complex instruction including the steps of: generating a first address based on a first presumed prefix byte; generating a second address based on a second presumed prefix byte; selecting an entry point address from either the first address or the second address based on a state of a prefix byte of the complex instruction; and accessing a first microcode instruction of the microcode sequence using the entry point address.

The invention still further contemplates a microprocessor including an instruction cache, a first instruction unit coupled to the instruction cache, a microcode instruction unit coupled to the instruction cache, a decode unit coupled to the first instruction unit and the microcode instruction unit, and a functional unit coupled to the decode unit. The microcode instruction unit includes a storage device, a plurality of address generators, and a selection circuit. The storage device is configured to store a plurality of microcode instructions, wherein a subset of the plurality of microcode instructions effectuates a function of a complex instruction, and the complex instruction includes an opcode byte and a prefix byte. The plurality of address generators is configured to generate an address of a first microcode instruction of the plurality of microcode instructions that effectuate the function of the complex instruction, wherein a first address generator generates a first address based on the opcode byte and a first presumed prefix byte, and a second address generator generates a second address based on the opcode byte and a second presumed prefix byte. The selection circuit coupled to the plurality of address generators and to the storage device, wherein the selection circuit is configured to select the first address if the first presumed prefix byte corresponds to the prefix byte of the complex instruction, and the selection circuit is configured to select the second address if the second presumed prefix byte corresponds to the prefix byte of the complex instruction.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the accompanying drawings in which:

FIG. 1A is a diagram which illustrates the generic x86 instruction set format.

FIG. 1B is a diagram which illustrates several different variable byte-length x86 instruction formats.

FIG. 1C is a diagram which illustrates several possible x86 addressing mode formats.

FIG. 2 is a block diagram of one embodiment of a superscalar microprocessor.

FIG. 3 is a block diagram depicting a portion of an instruction processing pipeline employed by one embodiment of the microprocessor shown in FIG. 2.

FIG. 4A is a block diagram of one embodiment of an MROM entry point circuit.

FIG. 4B is a block diagram of another embodiment of an MROM entry point circuit.

FIG. 5 is a flowchart illustrating operation of one embodiment an MROM entry point circuit.

FIG. 6 is a block diagram of one embodiment of a pair of decode units shown in FIG. 2.

FIG. 7 is a block diagram of one embodiment of an instruction cache and instruction alignment unit shown in FIG. 2.

FIG. 8 is a block diagram of a computer system including the microprocessor shown in FIG. 2.

While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to FIG. 2, a block diagram of one embodiment of a microprocessor 10 is shown. Microprocessor 10 includes a prefetch/predecode unit 12, a branch prediction unit 14, an instruction cache 16, an instruction alignment unit 18, a plurality of decode units 20A-20C, a plurality of reservation stations 22A-22C, a plurality of functional units 24A-24C, a load/store unit 26, a data cache 28, a register file 30, a reorder buffer 32, and an MROM unit 34. Blocks referred to herein with a reference number followed by a letter will be collectively referred to by the reference number alone. For example, decode units 20A-20C will be collectively referred to as decode units 20.

Prefetch/predecode unit 12 is coupled to receive instructions from a main memory subsystem (not shown), and is further coupled to instruction cache 16 and branch prediction unit 14. Similarly, branch prediction unit 14 is coupled to instruction cache 16. Still further, branch prediction unit 14 is coupled to decode units 20 and functional units 24. Instruction cache 16 is further coupled to MROM unit 34 and instruction alignment unit 18. Instruction alignment unit 18 is in turn coupled to decode units 20. Each decode unit 20A-20C is coupled to load/store unit 26 and to respective reservation stations 22A-22C. Reservation stations 22A-22C are further coupled to respective functional units 24A-24C. Additionally, decode units 20 and reservation stations 22 are coupled to register file 30 and reorder buffer 32. Functional units 24 are coupled to load/store unit 26, register file 30, and reorder buffer 32 as well. Data cache 28 is coupled to load/store unit 26 and to the main memory subsystem. Finally, MROM unit 34 is coupled to decode units 20.

Generally speaking, microprocessor 10 categorizes instructions as either MROM instructions or fastpath instructions. Generally speaking, fastpath instructions are defined as instructions that can be directly decoded by decode units 20. The remaining instructions are referred to as MROM instructions. MROM instructions are executed by invoking MROM unit 34. MROM unit 34 parses and issues the instructions into a subset of defined microcode instructions to effectuate the desired operation. MROM unit 34 dispatches the subset of microcode instructions to decode units 20. The subset of microcode instructions to effectuate the desired operation of the MROM instruction is referred to as a microcode sequence. MROM unit 34 includes an MROM entry point and an MROM access. The MROM access stores a plurality of microcode instructions that effectuate the desired operation of the MROM instruction. The MROM entry point generates a first address, or entry point address, within the MROM access at which the first microcode instruction of a microcode sequence that effectuates the MROM instruction is stored.

In one embodiment, the MROM entry point generates the entry point address from the prefix bytes, the opcode bytes, and the ModR/hi byte. In one particular embodiment, a plurality of prefix byte combinations affect the entry point address of the MROM instruction. To expedite the generation of the entry point address, a plurality of entry point address generators generate an entry point based on the opcode bytes, ModR/M byte and a presumed prefix byte. In other words, a first entry point generator generates an entry point from the opcode and ModR/M bytes presuming a first prefix byte combination. The other entry point generators generate an entry point from the opcode and ModR/M bytes presuming other prefix byte combinations. In parallel with the generation of the entry point, the prefix bytes are decoded to determine the actual prefix byte combination. Based on the output of the prefix decoder, the output of the entry point address generator that generated an address based on the actual prefix byte combination is selected.

Microprocessor 10 is configured to align instructions from instruction cache 16 to decode units 20 using instruction alignment unit 18. Instructions are fetched as an aligned plurality of bytes from a cache line within instruction cache 16. Instructions of interest may be stored beginning at any arbitrary byte within the fetched bytes. For example, a branch instruction may be executed having a target address which lies within a cache line. The instructions of interest therefore begin at the byte identified by the target address of the branch instruction. From the instruction bytes fetched, instruction alignment unit 18 identifies the instructions to be executed. Instruction alignment unit 18 conveys the instructions, in predicted program order, to decode units 20 for decode and execution.

Instruction alignment unit 18 includes a byte queue configured to store instruction bytes. An instruction scanning unit within instruction cache 16 separates the instructions fetched into instruction blocks. Each instruction block comprises a predefined number of instruction bytes. The instruction scanning unit identifies up to a predefined maximum number of instructions within the instruction block. Instruction identification information for each of the identified instructions is conveyed to instruction alignment unit 18 and is stored in the byte queue. The instruction identification information includes an indication of the validity of the instruction, as well as indications of the start and end of the instruction within the predefined number of instruction bytes. In one embodiment, the predefined number of instruction bytes comprises eight instruction bytes stored in contiguous main memory storage locations. The eight instruction bytes are aligned to an eight byte boundary (i.e. the least significant three bits of the address of the first of the contiguous bytes are zero). If more than the maximum number of instructions are contained within a particular predefined number of instruction bytes, the instruction bytes are scanned again during a subsequent clock cycle. The same instruction bytes are conveyed as another instruction block, with the additional instructions within the instruction bytes identified by the accompanying instruction identification information. Therefore, an instruction block may be defined as up to a predefined maximum number of instructions contained within a predefined number of instruction bytes.

The byte queue stores each instruction block and corresponding instruction identification information within a subqueue defined therein. The subqueues include a position for each possible valid instruction within the instruction block. The positions store instruction identification information and are maintained such that the instruction identification information for the first valid instruction within the subqueue is stored in a first position within the subqueue, instruction identification information regarding the second valid instruction (in program order) is stored in a second position within the subqueue, etc. When instructions within the subqueue are dispatched, instruction identification information corresponding to subsequent instructions are shifted within the positions of the subqueue such that the first of the remaining instructions is stored in the first position. Advantageously, instruction alignment unit 18 may only consider the instruction information stored in the first position of each subqueue to detect the instruction to be dispatched to decode unit 20A. Similarly, only the second position of the first subqueue (the subqueue storing instructions prior to the instructions stored in the other subqueues in program order) may be considered for dispatch of instructions to decode unit 20B. By managing the subqueues in this manner, logic for selecting and aligning instructions may be simplified. Fewer cascaded levels of logic may be employed for performing the selection and alignment process, allowing for high frequency implementation of microprocessor 10.

Because instructions are variable length, an instruction may begin within a particular instruction block but end in another instruction block. Instructions beginning within a particular instruction block and ending in another instruction block are referred to as "overflow instructions". The subqueue storing the instruction block within which an overflow instruction begins uses the last position to store the overflow instruction's identification information. Unlike the other positions, the instruction identification information of the last position is not shifted from the last position when an overflow instruction is stored therein. Advantageously, instruction alignment unit 18 need only search the last position of a particular subqueue to identify an instruction overflowing from one subqueue to another.

As used herein, the term queue refers to a storage device for storing a plurality of data items. The data items are stored with an ordered relationship between them. For example, the data items of the byte queue are instructions. The ordered relationship between the instructions is the program order of the instructions. Data items are removed from the queue according to the ordered relationship in a first in-first out (FIFO) fashion. Additionally, the term shifting is used to refer to movement of data items within the queue. When a data item is shifted from a first storage location to a second storage location, the data item is copied from the first storage location to the second storage location and invalidated in the second storage location. The invalidation may occur by shifting yet another data item into the second storage location, or by resetting a valid indication in the second storage location.

Instruction cache 16 is a high speed cache memory provided to store instructions. Instructions are fetched from instruction cache 16 and dispatched to decode units 20. In one embodiment, instruction cache 16 is configured to store up to 32 kilobytes of instructions in an 8 way set associative structure having 32 byte lines (a byte comprises 8 binary bits). Instruction cache 16 may additionally employ a way prediction scheme in order to speed access times to the instruction cache. Instead of accessing tags identifying each line of instructions and comparing the tags to the fetch address to select a way, instruction cache 16 predicts the way that is accessed. In this manner, the way is selected prior to accessing the instruction storage. The access time of instruction cache 16 may be similar to a direct-mapped cache. A tag comparison is performed and, if the way prediction is incorrect, the correct instructions are fetched and the incorrect instructions are discarded. It is noted that instruction cache 16 may be implemented as a fully associative, set associative, or direct mapped configuration.

Instructions are fetched from main memory and stored into instruction cache 16 by prefetch/predecode unit 12. Instructions may be prefetched prior to instruction cache 16 recording a miss for the instructions in accordance with a prefetch scheme. A variety of prefetch schemes may be employed by prefetch/predecode unit 12. As prefetch/predecode unit 12 transfers instructions from main memory to instruction cache 16, prefetch/predecode unit 12 generates three predecode bits for each byte of the instructions: a start bit, an end bit, and a functional bit. The predecode bits form tags indicative of the boundaries of each instruction. The predecode tags may also convey additional information such as whether a given instruction can be decoded directly by decode units 20 or whether the instruction is executed by invoking a microcode procedure controlled by MROM unit 34, as will be described in greater detail below. Still further, prefetch/predecode unit 12 may be configured to detect branch instructions and to store branch prediction information corresponding to the branch instructions into branch prediction unit 14.

One encoding of the predecode tags for an embodiment of microprocessor 10 employing the x86 instruction set will next be described. If a given byte is the first byte of an instruction, the start bit for that byte is set. If the byte is the last byte of an instruction, the end bit for that byte is set. For this embodiment of microprocessor 10, instructions which may be directly decoded by decode units 20 are referred to as "fast path" instructions. Fast path instructions may be an example of directly-decoded instructions for this embodiment. The remaining x86 instructions are referred to as MROM instructions, according to one embodiment. For this embodiment, MROM instructions are an example of microcode instructions.

For fast path instructions, the functional bit is set for each prefix byte included in the instruction, and cleared for other bytes. Alternatively, for MROM instructions, the functional bit is cleared for each prefix byte and set for other bytes. The type of instruction may be determined by examining the functional bit corresponding to the end byte. If that functional bit is clear, the instruction is a fast path instruction. Conversely, if that functional bit is set, the instruction is an MROM instruction. The opcode of an instruction may thereby be located within an instruction which may be directly decoded by decode units 20 as the byte associated with the first clear functional bit in the instruction. For example, a fast path instruction including two prefix bytes, a Mod R/M byte, and an SIB byte would have start, end, and functional bits as follows:

Start bits 10000

End bits 00001

Functional bits 11000

MROM instructions are instructions which are determined to be too complex for decode by decode units 20. MROM instructions are executed by invoking MROM unit 34. More specifically, when an MROM instruction is encountered, MROM unit 34 parses and issues the instruction into a subset of defined fast path instructions to effectuate the desired operation. MROM unit 34 dispatches the subset of fast path instructions to decode units 20. A listing of exemplary x86 instructions categorized as fast path instructions will be provided further below.

Microprocessor 10 employs branch prediction in order to speculatively fetch instructions subsequent to conditional branch instructions. Branch prediction unit 14 is included to perform branch prediction operations. In one embodiment, up to two branch target addresses are stored with respect to each cache line in instruction cache 16. Prefetch/predecode unit 12 determines initial branch targets when a particular line is predecoded. Subsequent updates to the branch targets corresponding to a cache line may occur due to the execution of instructions within the cache line. Instruction cache 16 provides an indication of the instruction address being fetched, so that branch prediction unit 14 may determine which branch target addresses to select for forming a branch prediction. Decode units 20 and functional units 24 provide update information to branch prediction unit 14. Because branch prediction unit 14 stores two targets per cache line, some branch instructions within the line may not be stored in branch prediction unit 14. Decode units 20 detect branch instructions which were not predicted by branch prediction unit 14. Functional units 24 execute the branch instructions and determine if the predicted branch direction is incorrect. The branch direction may be "taken", in which subsequent instructions are fetched from the target address of the branch instruction. Conversely, the branch direction may be "not taken", in which subsequent instructions are fetched from memory locations consecutive to the branch instruction. When a mispredicted branch instruction is detected, instructions subsequent to the mispredicted branch are discarded from the various units of microprocessor 10. A variety of suitable branch prediction algorithms may be employed by branch prediction unit 14.

Instructions fetched from instruction cache 16 are conveyed to instruction alignment unit 18. As instructions are fetched from instruction cache 16, the corresponding predecode data is scanned to provide information to instruction alignment unit 18 (and to MROM unit 34) regarding the instructions being fetched. Instruction alignment unit 18 utilizes the scanning data to align an instruction to each of decode units 20. In one embodiment, instruction alignment unit 18 aligns instructions from three sets of eight instruction bytes to decode units 20. Decode unit 20A receives an instruction which is prior to instructions concurrently received by decode units 20B and 20C (in program order). Similarly, decode unit 20B receives an instruction which is prior to the instruction concurrently received by decode unit 20C in program order. As used herein, the term "program order" refers to the order of the instruction as coded in the original sequence in memory. The program order of instructions is the order in which the instructions would be executed upon a microprocessor which fetches, decodes, executes, and writes the result of a particular instruction prior to fetching another instruction. Additionally, the term "dispatch" is used to refer to conveyance of an instruction to an issue position which is to execute the instruction. Issue positions may also dispatch load/store memory operations to load/store unit 26.

Decode units 20 are configured to decode instructions received from instruction alignment unit 18. Register operand information is detected and routed to register file 30 and reorder buffer 32. Additionally, if the instructions require one or more memory operations to be performed, decode units 20 dispatch the memory operations to load/store unit 26. Each instruction is decoded into a set of control values for functional units 24, and these control values are dispatched to reservation stations 22 along with operand address information and displacement or immediate data which may be included with the instruction.

Microprocessor 10 supports out of order execution, and thus employs reorder buffer 32 to keep track of the original program sequence for register read and write operations, to implement register renaming, to allow for speculative instruction execution and branch misprediction recovery, and to facilitate precise exceptions. A temporary storage location within reorder buffer 32 is reserved upon decode of an instruction that involves the update of a register to thereby store speculative register states. If a branch prediction is incorrect, the results of speculatively-executed instructions along the mispredicted path can be invalidated in the buffer before they are written to register file 30. Similarly, if a particular instruction causes an exception, instructions subsequent to the particular instruction may be discarded. In this manner, exceptions are "precise" (i.e. instructions subsequent to the particular instruction causing the exception are not completed prior to the exception). It is noted that a particular instruction is speculatively executed if it is executed prior to instructions which precede the particular instruction in program order. Preceding instructions may be a branch instruction or an exception-causing instruction, in which case the speculative results may be discarded by reorder buffer 32.

The instruction control values and immediate or displacement data provided at the outputs of decode units 20 are routed directly to respective reservation stations 22. In one embodiment, each reservation station 22 is capable of holding instruction information (i.e., instruction control values as well as operand values, operand tags and/or immediate data) for up to three pending instructions awaiting issue to the corresponding functional unit. It is noted that for the embodiment of FIG. 1, each reservation station 22 is associated with a dedicated functional unit 24. Accordingly, three dedicated "issue positions" are formed by reservation stations 22 and functional units 24. In other words, issue position 0 is formed by reservation station 22A and functional unit 24A. Instructions aligned and dispatched to reservation station 22A are executed by functional unit 24A. Similarly, issue position 1 is formed by reservation station 22B and functional unit 24B; and issue position 2 is formed by reservation station 22C and functional unit 24C. As used herein, the term "issue position" refers to logic circuitry configured to receive an instruction and to execute that instruction. Once the instruction enters the issue position, it remains in that issue position until the execution of the instruction is completed.

Upon decode of a particular instruction, if a required operand is a register location, register address information is routed to reorder buffer 32 and register file 30 simultaneously. Those of skill in the art will appreciate that the x86 register file includes eight 32 bit real registers (i.e., typically referred to as EAX, EBX, ECX, EDX, EBP, ESI, EDI and ESP). In embodiments of microprocessor 10 which employ the x86 microprocessor architecture, register file 30 comprises storage locations for each of the 32 bit real registers. Additional storage locations may be included within register file 30 for use by MROM unit 34. Reorder buffer 32 contains temporary storage locations for results which change the contents of these registers to thereby allow out of order execution. A temporary storage location of reorder buffer 32 is reserved for each instruction which, upon decode, is determined to modify the contents of one of the real registers. Therefore, at various points during execution of a particular program, reorder buffer 32 may have one or more locations which contain the speculatively executed contents of a given register. If following decode of a given instruction it is determined that reorder buffer 32 has a previous location or locations assigned to a register used as an operand in the given instruction, the reorder buffer 32 forwards to the corresponding reservation station either: 1) the value in the most recently assigned location, or 2) a tag for the most recently assigned location if the value has not yet been produced by the functional unit that will eventually execute the previous instruction. If reorder buffer 32 has a location reserved for a given register, the operand value (or reorder buffer tag) is provided from reorder buffer 32 rather than from register file 30. If there is no location reserved for a required register in reorder buffer 32, the value is taken directly from register file 30. If the operand corresponds to a memory location, the operand value is provided to the reservation station through load/store unit 26.

In one particular embodiment, reorder buffer 32 is configured to store and manipulate concurrently decoded instructions as a unit. This configuration will be referred to herein as "line-oriented". By manipulating several instructions together, the hardware employed within reorder buffer 32 may be simplified. For example, a line-oriented reorder buffer included in the present embodiment allocates storage sufficient for instruction information pertaining to three instructions (one from each decode unit 20) whenever one or more instructions are dispatched by decode units 20. By contrast, a variable amount of storage is allocated in conventional reorder buffers, dependent upon the number of instructions actually dispatched. A comparatively larger number of logic gates may be required to allocate the variable amount of storage. When each of the concurrently decoded instructions has executed, the instruction results are stored into register file 30 simultaneously. The storage is then free for allocation to another set of concurrently decoded instructions. Additionally, the amount of control logic circuitry employed per instruction is reduced because the control logic is amortized over several concurrently decoded instructions. A reorder buffer tag identifying a particular instruction may be divided into two fields: a line tag and an offset tag. The line tag identifies the set of concurrently decoded instructions including the particular instruction, and the offset tag identifies which instruction within the set corresponds to the particular instruction. It is noted that storing instruction results into register file 30 and freeing the corresponding storage is referred to as "retiring" the instructions. It is further noted that any reorder buffer configuration may be employed in various embodiments of microprocessor 10.

As noted earlier, reservation stations 22 store instructions until the instructions are executed by the corresponding functional unit 24. An instruction is selected for execution if: (i) the operands of the instruction have been provided; and (ii) the operands have not yet been provided for instructions which are within the same reservation station 22A-22C and which are prior to the instruction in program order. It is noted that when an instruction is executed by one of the functional units 24, the result of that instruction is passed directly to any reservation stations 22 that are waiting for that result at the same time the result is passed to update reorder buffer 32 (this technique is commonly referred to as "result forwarding"). An instruction may be selected for execution and passed to a functional unit 24A-24C during the clock cycle that the associated result is forwarded. Reservation stations 22 route the forwarded result to the functional unit 24 in this case.

In one embodiment, each of the functional units 24 is configured to perform integer arithmetic operations of addition and subtraction, as well as shifts, rotates, logical operations, and branch operations. The operations are performed in response to the control values decoded for a particular instruction by decode units 20. It is noted that a floating point unit (not shown) may also be employed to accommodate floating point operations. The floating point unit may be operated similar to load/store unit 26 in that any of decode units 20 may dispatch instructions to the floating point unit. Additionally, functional units 24 may be configured to perform address generation for load and store memory operations performed by load/store unit 26.

Each of the functional units 24 also provides information regarding the execution of conditional branch instructions to the branch prediction unit 14. If a branch prediction was incorrect, branch prediction unit 14 flushes instructions subsequent to the mispredicted branch that have entered the instruction processing pipeline, and causes fetch of the required instructions from instruction cache 16 or main memory. It is noted that in such situations, results of instructions in the original program sequence which occur after the mispredicted branch instruction are discarded, including those which were speculatively executed and temporarily stored in load/store unit 26 and reorder buffer 32.

Results produced by functional units 24 are sent to reorder buffer 32 if a register value is being updated, and to load/store unit 26 if the contents of a memory location are changed. If the result is to be stored in a register, reorder buffer 32 stores the result in the location reserved for the value of the register when the instruction was decoded. A plurality of result buses 38 are included for forwarding of results from functional units 24 and load/store unit 26. Result buses 38 convey the result generated, as well as the reorder buffer tag identifying the instruction being executed.

Load/store unit 26 provides an interface between functional units 24 and data cache 28. In one embodiment, load/store unit 26 is configured with a load/store buffer having eight storage locations for data and address information for pending loads or stores. Decode units 20 arbitrate for access to the load/store unit 26. When the buffer is full, a decode unit must wait until load/store unit 26 has room for the pending load or store request information. Load/store unit 26 also performs dependency checking for load memory operations against pending store memory operations to ensure that data coherency is maintained. A memory operation is a transfer of data between microprocessor 10 and the main memory subsystem. Memory operations may be the result of an instruction which utilizes an operand stored in memory, or may be the result of a load/store instruction which causes the data transfer but no other operation. Additionally, load/store unit 26 may include a special register storage for special registers such as the segment registers and other registers related to the address translation mechanism defined by the x86 microprocessor architecture.

In one embodiment, load/store unit 26 is configured to perform load memory operations speculatively. Store memory operations are performed in program order, but may be speculatively stored into the predicted way. If the predicted way is incorrect, the data prior to the store memory operation is subsequently restored to the predicted way and the store memory operation is performed to the correct way. In another embodiment, stores may be executed speculatively as well. Speculatively executed stores are placed into a store buffer, along with a copy of the cache line prior to the update. If the speculatively executed store is later discarded due to branch misprediction or exception, the cache line may be restored to the value stored in the buffer. It is noted that load/store unit 26 may be configured to perform any amount of speculative execution, including no speculative execution.

Data cache 28 is a high speed cache memory provided to temporarily store data being transferred between load/store unit 26 and the main memory subsystem. In one embodiment, data cache 28 has a capacity of storing up to sixteen kilobytes of data in an eight way set associative structure. Similar to instruction cache 16, data cache 28 may employ a way prediction mechanism. It is understood that data cache 28 may be implemented in a variety of specific memory configurations, including a set associative configuration.

In one particular embodiment of microprocessor 10 employing the x86 microprocessor architecture, instruction cache 16 and data cache 28 are linearly addressed. The linear address is formed from the offset specified by the instruction and the base address specified by the segment portion of the x86 address translation mechanism. Linear addresses may optionally be translated to physical addresses for accessing a main memory. The linear to physical translation is specified by the paging portion of the x86 address translation mechanism. It is noted that a linear addressed cache stores linear address tags. A set of physical tags (not shown) may be employed for mapping the linear addresses to physical addresses and for detecting translation aliases. Additionally, the physical tag block may perform linear to physical address translation.

Turning now to FIG. 3, a diagram depicting portions of instruction cache 16, instruction alignment unit 18, decode unit 20 and MROM unit 34 is shown. Embodiments of microprocessor 10 may employ other elements in addition to those shown in FIG. 3. Instruction cache 16 includes instruction fetch 70 and instruction scan 72. MROM unit 34 includes MROM entry point 62, MROM access 64, sequence control 65, and MROM early decode 66. Instruction alignment unit 18 includes byte queue 74, selection control unit 76, and multiplex to issue 78. Decode unit 20 includes early decode 40 and opcode decode 44.

Instruction fetch 70 fetches instructions from instruction cache storage (not shown) using addresses provided by branch prediction unit 14. Instruction fetch 70 conveys the fetched instructions to instruction scan unit 72. Instruction scan unit 72 conveys the instructions to byte queue 74, and detects MROM instructions. MROM instructions detected by instruction scan unit 72 are conveyed to MROM entry point 62. In one embodiment, one MROM instruction per clock cycle is accepted by MROM unit 34. Therefore, if a second MROM instruction is detected within a set of instruction bytes being scanned during a particular clock cycle, instruction blocks including the second MROM instruction and subsequent instructions in the program order are stalled until a subsequent clock cycle.

The flow of instructions through instruction alignment unit 18 and decode unit 20 is discussed in more detail below with reference to FIG. 7. The flow of instructions through MROM unit 34 is discussed next. MROM entry point 62 calculates the location, or microaddress, of the first microcode instruction in MROM access 64 that corresponds to an MROM instruction received from instruction scan unit 72. In one embodiment, the microaddress is calculated from the opcode bytes, the ModR/M byte, and the prefix bytes of the MROM instruction. The microaddress calculated by MROM entry point 62 is the location of the first microcode line that stores the microcode instructions that effectuate the desired operation of the MROM instruction. Microcode instructions, like fastpath instructions, are directly decodeable by the decode unit. The number of microcode instructions required to implement an MROM instruction varies from MROM instruction to MROM instruction. Complex MROM instructions may require many microcode instructions, and relatively simple MROM instructions may be implemented by two microcode instructions. The microcode instructions that implement an MROM instruction may include branch instructions. For example, in one embodiment the microcode instructions to implement the MROM instruction MOVS are:

    ______________________________________                                                LDDF               ;load direction flag to latch in FU                         OR        ecx,ecx  ;test if ecx is zero                                        JZ        end.sub.-- loop                                                                         ;terminate string moves if                                                     ecx is zero                                          loop:  MOVFM+    tmp0,    ;move to tmp0 data from                                                esi!    source and inc/dec esi                                      MOVTM+     edi!,   ;move the data to                                                     tmp0     destination and inc/dec edi                                 DECXJNZ   loop     ;dec ecx and repeat until zero                       end.sub.-- loop:                                                                      EXIT                                                                    ______________________________________                                    

MROM access 64 is a storage device capable of storing microcode instructions. In one embodiment, MROM access 64 is a read-only memory (ROM). In other embodiments, other storage devices can be used to implement MROM access 64. MROM access 64 uses the entry point microaddress generated by MROM entry point 62 to access the first microcode instruction line. In one embodiment, MROM access 64 stores multiple microcode instructions in each microcode line of MROM access 64. In one specific embodiment, each microcode line contains a number of microcode instructions equal to the number of functional units in the microprocessor. Because the number of microcode instructions required to implement an MROM instruction may exceed the number of microcode instructions in one line of MROM access 64, sequence control 65 determines the location of the next line of microcode to be accessed. The next line of microcode to be accessed may be the subsequent line in MROM access 64. If, however, a microcode line in MROM access 64 includes a "taken" branch instruction, sequence control unit 65 detects the branch microaddress and provides that microaddress as the next line to access from MROM access 64. MROM access 64 may contain two types of branch instructions: conditional sequence control branches and predict branches. A conditional sequence control branch is a branch instruction in which the branch condition can be resolved during decode. Predict branches are branches that cannot be resolved until the branch instruction is executed by a functional unit. Predict branches may depend on the state on the EFLAGs register. Because dispatched instructions that have not completed execution may modify the contents of this register, the branch condition cannot be resolved during decode. Therefore, a prediction is made whether the branch is "taken" or "not taken". If the branch is predicted "taken", sequence control 65 generates the microaddress of the next microcode line to fetch from a branch offset address. If the branch is predicted "not taken", sequence control 65 provides the microaddress of the next microcode line as the next microcode line to fetch. In one embodiment, MROM unit 34 uses static branch prediction. In other words, the prediction is made during coding of the microcode and does not change. Other embodiments may use more complex branch prediction algorithms.

In addition to specifying branch addresses, sequence control 65 also detects the last line of the microcode sequence. If a line of microcode in MROM access 64 is the last line of microcode to implement an MROM instruction, sequence control 65 selects the entry point of the next MROM instruction, provided by MROM entry point 62, as the microaddress of the next line of microcode.

Each line of microcode fetched from MROM access 64 is dispatched to MROM early decode 66. MROM early decode 66 formats the instructions similar to the formatting of early decode unit 40 discussed in more detail with reference to FIG. 6. The formatted instructions are conveyed to opcode decode 44, which selects either the microcode instructions conveyed by MROM early decode 66 or fastpath instructions conveyed by early decode 40.

Fastpath instructions from instruction alignment unit 18 are stalled while MROM microcode instructions that implement an MROM instruction are issued by MROM unit 34. It is noted that a particular MROM instruction may arrive at MROM access 64 prior to being selected for dispatch from byte queue 74. The particular MROM instruction may be subsequent to a large number of fast path instructions within the byte queue and instructions are selected for dispatch in program order. MROM instructions are routed to MROM unit 34 but are not removed from the instruction blocks conveyed to instruction alignment unit 18. For these reasons, synchronization is provided between MROM access 64 and multiplex to issue 78. When MROM access 64 receives an entry point address from MROM entry point 62, MROM access 64 informs multiplex to issue 78 by asserting a signal upon synchronization bus 67. When multiplex to issue 78 receives a dispatched MROM instruction from byte queue 74, multiplex to issue 78 signals MROM access 64 via synchronization bus 67. In this manner, the MROM instruction progresses to both MROM early decode 66 and early decode 40 during the same clock cycle. Because both MROM access 64 and multiplex to issue 78 receive instructions in program order it is sufficient to synchronize instructions via synchronization bus 67.

As discussed above, sequence control 65 determines the microaddress of the next microcode line to fetch from MROM access 64. To generate the next address, each microcode line has a microcode sequence control field, or simply "sequence control field", associated with it. In one embodiment, this field contains data indicating whether the line is the last line of the microcode instruction, whether the microcode line contains a jump, call or branch, conditions for conditional microbranches, and other information necessary for sequence control of MROM access 64. In one particular embodiment, the sequence control field for a microcode line is appended to the previous microcode line. In this manner, the sequence control field is known one cycle in advance. Therefore, the generation of the next address is done in parallel with accessing the microcode line from MROM access 64.

Sequence control 65 also detects the last microcode line in a microcode sequence that implements an MROM instruction. In one particular embodiment, the sequence control field 65 appended to the microcode line prior to the last microcode line indicates that the subsequent microcode line is the last microcode line. This indication, called an early exit signal, is used by sequence control 65 to select the entry point generated by MROM entry point 62 as the microaddress from which to fetch a microcode line following the subsequent line. In one embodiment, in addition to indicating that the subsequent microcode line is the last microcode line of an MROM instruction, the sequence control field indicates how many microcode instructions are in the last microcode line. In one embodiment, the number of microcode instructions in the last microcode line is conveyed to selection control 76 via an early exit signal line 69. Selection control 76 uses the information from the early exit signal to pack additional fastpath instructions at the end of the microcode instruction line. In this manner, instructions are issued to each functional unit and dispatch bandwidth is not wasted.

FIG. 4A illustrates one embodiment of MROM entry point 62. MROM entry point 62 includes an entry point address generator 402A, an entry point address generator 402B, a prefix decoder 406, and a multiplexor 408. Entry point address generators 402A and 402B are configured to receive opcode and ModR/M bytes from instruction scan 72. Prefix decoder 406 is configured to receive prefix bytes from instruction scan 72. The inputs of multiplexor 408 are coupled to the outputs of entry point address generators 402A and 402B. A control line of multiplexor 408 is coupled to the output of prefix decoder 406.

In one embodiment, the entry point address generated by MROM entry point 62 is dependent on the prefix bytes, the opcode bytes, and the ModR/M byte of the MROM instruction. MROM entry point 62 receives these instruction bytes from instruction scan 72. In one particular embodiment, one particular prefix byte code combination affects the entry point address generated by MROM entry point 62. This particular prefix byte code combination is 0F (hex). Other prefix bytes may be used to change the address or operand size of an instruction, or to override the default segment used in memory addressing. In this particular embodiment, only one prefix byte code combination is of interest to MROM entry point 62. MROM entry point 62 must determine whether the prefix bytes contains the code 0F or not. MROM instructions that do not have prefix bytes are treated as MROM instructions with prefix bytes that do not contain the code 0F.

To expedite the generation of an entry point, the prefix byte is decoded in parallel with the generation of a plurality of entry point addresses. Each entry point address generator generates an entry point based on a presumed prefix byte combination. In the embodiment illustrated in FIG. 4A, two entry point address generators are used. Entry point generator 402A calculates an entry point from the opcode and ModR/M bytes based on the presumption that the prefix bytes contain the code 0F. Entry point generator 402B generates an entry point from the opcode and ModR/M bytes based on the presumption that the prefix bytes do not contain the code 0F. As discussed above, in one particular embodiment, the prefix byte combination of 0F affects entry point address generation. One of the two entry points generated by entry points address generators 402A and 402B is the proper entry point address. In parallel with the generation of the entry point address by entry point address generators 402A and 402B, prefix decoder 406 decodes the prefix bytes to determine whether the prefix bytes contain the code 0F or not. Prefix decoder 406 outputs a prefix signal on a prefix signal line 410. Generally speaking, the prefix signal is indicative of the prefix byte combination of the prefix bytes. In one particular embodiment, the prefix signal is indicative of whether the prefix bytes contain the code 0F or not.

Multiplexor 408 is coupled to the outputs of the plurality of entry point generators. Multiplexor 408 is further coupled to prefix signal line 410. Based on the state of the prefix signal, multiplexor 408 selects the output of the entry point address generator that calculated the entry point based on the correct prefix code. In the embodiment shown in FIG. 4A, multiplexor 408 is coupled to the outputs of entry point address generators 402A and 402B. If the prefix signal indicates that the prefix bytes contains the code 0F, multiplexor 408 selects the output of entry point generator 402A, which generated an entry point based on the prefix byte containing the code 0F. If the prefix signal indicates that the prefix byte did not contain the code 0F, or no prefix byte was present, multiplexor 408 selects the output of entry point generator 402B, which generated an entry point based on the prefix byte not containing the code 0F. The entry point is conveyed to MROM access 64 which uses the entry point as the address of the first microcode instruction of the microcode sequence that effectuates the function of the MROM instruction.

In one embodiment, entry point address generators 402 are implemented using programmable logic arrays. In other embodiments, entry point address generators 402 may be implemented using combinational logic or other suitable methods for decoding or translating digital data. In one embodiment, prefix decoder 406 is implemented using combinational logic. In other embodiments, prefix decoder 406 is implemented using other suitable methods for decoding digital data.

FIG. 4B illustrates an alternative embodiment of MROM entry point 62. MROM entry point 62 includes a plurality of entry point address generators 402A through 402D, a prefix decoder 406, and a multiplexor 408. Entry point address generators 402 are configured to receive opcode and ModR/M bytes from instruction scan 72. Prefix decoder 402 is configured to receive prefix bytes from instruction scan 72. The inputs of multiplexor 408 are coupled to the outputs of entry point address generators 402. A control signal of multiplexor 408 is coupled to the output of prefix decoder 406. In one embodiment, the entry point address generated by MROM entry point 62 is dependent on the prefix bytes, the opcode bytes, and the ModR/M byte of the MROM instruction. In one particular embodiment, a plurality of prefix byte code combinations affect the entry point address generated by MROM entry point 62. In one particular embodiment, the prefix byte combinations that affect the entry point address generation are grouped into three categories. The first category is floating point escape prefix byte combinations. The floating point escape prefix byte combinations in one embodiment are D8 through DF. The second category of prefix byte combinations that affect entry point addresses are the repeat prefix bytes (REP/REPE/REPNE). The repeat prefix byte combinations are represented by the prefix byte combinations F2 and F3. The third category of prefix byte combinations that affect entry point address generation is the prefix byte combination 0F. MROM instructions that do not have a prefix bytes are treated as MROM instructions with prefix byte combinations that do not fall into one of the three identified prefix byte categories.

To expedite the generation of an entry point, the prefix bytes are decoded in parallel with the generation of a plurality of entry point addresses. Each entry point address generator generates an entry point based on a presumed prefix byte combination. In the embodiment illustrated in FIG. 4B, four address entry point generators are used. Entry point address generator 402A calculates an entry point from the opcode ModR/M bytes based on the presumption that the prefix bytes contain a floating point escape prefix byte, combination. Entry point generator 402B generates an entry point from the opcode and ModR/M bytes based on the presumption that the prefix bytes contain a repeat prefix byte combination. Entry point generator 402C generates an entry point from the opcode and ModR/M bytes based on the presumption of the prefix bytes contained in the code 0F. Entry point generator 402D generates an entry point from the opcode and ModR/M bytes based on the presumption that the prefix bytes do not contain either a floating point escape, repeat or 0F prefix byte combination. One of the four entry points generated by entry point address generators 402 is the proper entry point address. In parallel with the generation of the entry point address by entry point address generators 402, prefix decoder 406 decodes the prefix bytes to determine the actual prefix byte combination. Prefix decoder 406 outputs a prefix signal on a prefix signal line 410 selecting the proper entry point address.

FIG. 5 is a flow chart illustrating the operation of one embodiment of MROM entry point 62. In parallel, two entry point addresses are generated and prefix bytes of an MROM instruction are decoded. In a step 502, a first entry point address is generated based on the prefix bytes containing the code 0F. In a step 504, a second entry point address is generated based on the prefix bytes not containing the code 0F. In a step 506, the prefix bytes are decoded to determine whether the prefix bytes contain the code 0F. In a decisional step 508, it is determined whether the prefix bytes contain the code 0F. If the prefix bytes contain the code 0F, in a step 510 the entry point address generated in step 502 is selected. If the prefix bytes do not contain the code 0F, in a step 512 the entry point address generated in step 504 is selected. If no prefix byte is present in the MROM instruction, the MROM instruction is treated the same as if a prefix byte is present but does not contain the code 0F. In a step 514 the selected entry point address is used to access a microcode sequence in MROM access 64 that corresponds to the MROM instruction. In another embodiment, MROM entry point 62 generates four entry points in parallel and selects the proper entry point based on the decoded prefix bytes.

Turning now to FIG. 6, a block diagram of one embodiment of decode units 20B and 20C is shown. Each decode unit 20 receives an instruction from instruction alignment unit 18. Additionally, MROM unit 34 is coupled to each decode unit 20 for dispatching microcode instructions corresponding to a particular MROM instruction. Decode unit 20B comprises early decode unit 40B, multiplexor 42B, and opcode decode unit 44B. Similarly, decode unit 20C includes early decode unit 40C, multiplexor 42C, and opcode decode unit 44C.

Certain instructions in the x86 instruction set are both fairly complicated and frequently used. In one embodiment of microprocessor 10, such instructions include more complex operations than the hardware included within a particular functional unit 24A-24C is configured to perform. Some of such instructions are classified as a special type of MROM instruction referred to as a "double dispatch" instruction. These instructions are dispatched to a pair of opcode decode units 44 by MROM unit 34. It is noted that opcode decode units 44 are coupled to respective reservation stations 22. Each of opcode decode units 44A-44C forms an issue position with the corresponding reservation station 22A-22C and functional unit 24A-24C. Instructions are passed from an opcode decode unit 44 to the corresponding reservation station 22 and further to the corresponding functional unit 24.

Multiplexor 42B is included for selecting between the instructions provided by MROM unit 34 and by early decode unit 40B. During times in which MROM unit 34 is dispatching instructions, multiplexor 42B selects instructions provided by MROM unit 34. At other times, multiplexor 42B selects instructions provided by early decode unit 40B. Similarly, multiplexor 42C selects between instructions provided by MROM unit 34, early decode unit 40B, and early decode unit 40C. The instruction from MROM unit 34 is selected during times in which MROM unit 34 is dispatching instructions. During times in which early decode unit 40A detects a double dispatch instruction, the instruction from early decode unit 40B is selected by multiplexor 42C. Otherwise, the instruction from early decode unit 40C is selected. Selecting the instruction from early decode unit 40B into opcode decode unit 44C allows a fast path instruction decoded by decode unit 20B to be dispatched concurrently with a double dispatch instruction decoded by decode unit 20A. In this manner, instruction alignment unit 18 need not attempt to align MROM instructions and concurrently dispatched fast path instructions to their final issue positions. Instead, the instructions may be aligned to a position and then adjusted between early decode units 40 and opcode decode units 44.

According to one embodiment employing the x86 instruction set, early decode units 40 perform the following operations:

(i) merge the prefix bytes of the instruction into an encoded prefix byte;

(ii) decode unconditional branch instructions (which may include the unconditional jump, the CALL, and the RETURN) which were not detected during branch prediction;

(iii) decode source and destination flags;

(iv) decode the source and destination operands which are register operands and generate operand size information; and

(v) determine the displacement and/or immediate size so that displacement and immediate data may be routed to the opcode decode unit.

Opcode decode units 44 are configured to decode the opcode of the instruction, producing control values for functional unit 24. Displacement and immediate data are routed with the control values to reservation stations 22.

Since early decode units 40 detect operands, the outputs of multiplexors 42 are routed to register file 30 and reorder buffer 32. Operand values or tags may thereby be routed to reservation stations 22. Additionally, memory operands are detected by early decode units 40. Therefore, the outputs of multiplexors 42 are routed to load/store unit 26. Memory operations corresponding to instructions having memory operands are stored by load/store unit 26.

In one embodiment, MROM entry point 62 determines if a particular MROM instruction is double dispatch. A particular MROM instruction is double dispatch if the particular MROM instruction corresponds to a single line of instructions within which two instructions are stored. If MROM entry point 62 detects a double dispatch instruction, a double dispatch signal upon a double dispatch signal line 68 is asserted. Otherwise, the double dispatch signal is deasserted. The double dispatch signal is conveyed to selection control 76. Selection control 76 uses the state of the double dispatch signal to determine if instructions dispatched during the previous clock cycle should be discarded. More particularly, selection control 76 discards the second instruction in program order if: (i) an MROM instruction and a fast path instruction were concurrently dispatched; and (ii) the double dispatch signal is deasserted. Otherwise, selection control 76 passes the instructions to early decode stage 58 via multiplex to issue 78.

When byte queue 76 concurrently dispatches an MROM instruction and a fast path instruction (referred to herein as "packing"), the second of the two instructions in program order is retained in the byte queue. During each clock cycle, byte queue 74 initially selects up to four instructions for dispatch during a particular clock cycle. If byte queue 74 packed during the previous clock cycle and the double dispatch signal is asserted, then the first of the four instructions (in program order) is ignored and the remainder are dispatched. Conversely, if byte queue 74 did not pack during the previous clock cycle or the double dispatch signal is deasserted, the first three of the four instructions (in program order) are dispatched and the fourth is retained by the byte queue. In this manner, redispatch of the second of the packed instructions is performed when needed without sacrificing other dispatch positions.

Turning now to FIG. 7, a block diagram of one embodiment of instruction cache 16 and instruction alignment unit 18 are shown. Instruction cache 16 includes an instruction cache storage and control block 70 and an instruction scanning unit 72. Instruction alignment unit 18 includes a byte queue 74, a selection control unit 76, and a multiplex to issue block 78.

Instruction cache storage and control block 70 includes storage for instruction cache lines and related control circuitry for fetching instructions from the storage, for selecting cache lines to discard when a cache miss is detected,. etc. Instruction cache storage and control block 70 receives fetch addresses from branch prediction unit 14 (FIG. 1) in order to fetch instructions for execution by microprocessor 10. Instruction bytes fetched from instruction cache storage and control block 70 are conveyed to instruction scanning unit 72 upon an instructions bus 80. Instruction bytes are conveyed upon instructions bus 80, as well as corresponding predecode data (e.g. start, end, and functional bits). In one embodiment, sixteen bytes stored in contiguous memory locations are conveyed upon instructions bus 80 along with the corresponding predecode data. The sixteen bytes form either the upper or lower half of the 32 byte cache line. The upper half of the cache line is the half stored in memory addresses having larger numerical values, while the lower half is stored in memory addresses having smaller numerical values. Additionally, instruction scanning unit 72 receives information regarding the bytes within the sixteen bytes which are to be conveyed as instructions to instruction alignment unit 18. Instruction bytes at the beginning of the sixteen bytes may be ignored if the bytes are fetched as the target of a branch instruction, and the target address identifies a byte other than the first byte of the sixteen bytes. Additionally, if a branch instruction is within the sixteen bytes and branch prediction unit 14 predicts the branch taken, then bytes subsequent to the branch instruction within the sixteen bytes are ignored.

Instruction scanning unit 72 scans the predecode data associated with the bytes which are to be conveyed as instructions to instruction alignment unit 18. In the present embodiment, instruction scanning unit 72 divides the sixteen bytes conveyed by instruction cache storage and control block 70 into two portions comprising eight contiguous bytes each. One portion forms the lower half of the sixteen bytes (i.e. the bytes stored at smaller numerical addresses than the bytes forming the upper half of the sixteen bytes). The other portion forms the upper half of the sixteen bytes. Therefore, an eight byte portion forms one of four quarters of the 32 byte cache line employed by instruction cache storage and control block 70, according to the present embodiment. As used herein, bytes are contiguous if they are stored in contiguous memory locations in the main memory subsystem. It is noted that particular sizes of various components, such as instruction block sizes, are used herein for clarity of the description. Any size may be used for each component within the spirit and scope of the appended claims.

Instruction scanning unit 72 scans the predecode data of each portion of the instructions independently and in parallel. Instruction scanning unit 72 identifies up to a predefined maximum number of instructions within each portion from the start and end byte information included within the predecode data. For the present embodiment, the predefined maximum number is three. Generally speaking, instruction scanning unit 72 preferably identifies a maximum number of instructions in each portion equal to the number of issue positions included within microprocessor 10.

The instruction bytes and instruction identification information generated by instruction scanning unit 72 are conveyed to byte queue 74 upon an instructions bus 82 and an instruction data bus 84, respectively. The instruction bytes are conveyed as eight byte portions, and the instruction data is arranged accordingly such that each eight byte portion is associated with a portion of the instruction identification information conveyed upon instruction data bus 84. Each eight byte portion and the corresponding instruction identification information forms an instruction block. It is noted that, although an instruction block includes eight bytes in the present embodiment, instruction blocks may include any number of bytes in various embodiments. Byte queue 74 receives the instruction blocks conveyed and stores them into one of multiple subqueues included therein. In the embodiment shown, byte queue 74 includes three subqueues: a first subqueue 86A, a second subqueue 86B, and a third subqueue 86C. First subqueue 86A stores the instruction block which is foremost among the instruction blocks stored in byte queue 74 in program order. Second subqueue 86B stores the instruction block which is second in program order, and third subqueue stores the instruction block which is third in program order. It is noted that various embodiments of byte queue 74 may include any number of subqueues 66.

If a particular portion as scanned by instruction scanning unit 72 includes more than the maximum predefined number of instructions, then the particular portion is retained by instruction scanning unit 72. During the following clock cycle, the particular eight byte portion is scanned again. The predecode data corresponding to the previously identified instructions is invalidated such that instruction scanning unit 72 detects the additional instructions. If the other portion concurrently received with the particular portion is subsequent to the particular portion in program order, then the other portion is rescanned as well. Byte queue 74 discards the instruction blocks received from the other portion, in order to retain program order among the instruction blocks stored in the byte queue.

A control unit 90 within byte queue 74 conveys a byte queue status upon byte queue status bus 88 to instruction scanning unit 72. Byte queue status bus 88 includes a signal corresponding to each subqueue 86. The signal is asserted if the subqueue 86 is storing an instruction block, and deasserted if the subqueue 86 is not storing an instruction block. In this manner, instruction scanning unit 72 may determine how many instruction blocks are accepted by byte queue 74 during a clock cycle. If two instruction blocks are conveyed during a clock cycle and only one instruction block is accepted, instruction scanning unit 72 retains the rejected instruction block and rescans the instruction block in the subsequent clock cycle.

As noted above, an instruction block may contain up to a predefined maximum number of instructions (e.g. three in the present embodiment). Additionally, eight contiguous bytes are conveyed for each instruction block in the present embodiment. However, due to the variable byte length of the x86 instructions, an instruction may begin within one set of contiguous bytes and end in another set of contiguous bytes, referred to as overflow instructions. If an overflow instruction is detected, it is identified as the last of the predefined number of instructions. Instead of being indicated as a valid instruction within the instruction block, the overflow instruction is identified as an overflow. Instruction identification information is generated, but the instruction is handled somewhat differently, as will be explained in more detail below.

In one embodiment, the instruction identification information for each instruction includes: (i) start and end pointers identifying the bytes at which the identified instruction begins and ends within the eight bytes; (ii) a valid mask containing eight bits, one for each of the eight bytes; (iii) a bit indicative of whether the instruction is MROM or fast path; and (iv) an instruction valid bit indicating that the instruction is valid and an overflow bit for the last instruction indicating that it is an overflow. The valid mask includes a binary one bit corresponding to each byte included within the particular instruction (i.e. the bits between the start pointer and end pointer, inclusive, are set). Zero bits are included for the other bytes. Additional information conveyed with the instruction identification information is the taken/not taken prediction if the instruction is a branch instruction, bits indicating which of the quarters of the 32 byte cache line the eight bytes correspond to, the functional bits from the predecode data corresponding to the eight bytes, and a segment limit identifying the segment limit within the eight bytes for exception handling. The additional information is provided by instruction cache storage and control block 70 except for the branch prediction, which is provided by branch prediction unit 14.

Selection control unit 76 examines the instruction identification information stored in each subqueue to generate selection controls for multiplex to issue block 78. Multiplex to issue block 78 includes a plurality of multiplexors for selecting instruction bytes from byte queue 74 for conveyance to each of decode units 20. Byte queue 74 maintains certain properties with respect to each subqueue 86 in order to simplify the selection logic within selection control unit 76, as will be explained in more detail below. Instructions are selected and conveyed, and corresponding instruction identification information is invalidated such that subsequent instructions may be dispatched in subsequent clock cycles.

Subqueues 86 store instruction information in a plurality of instruction positions (or simply "positions"). The number of instruction positions is preferably equal to the maximum number of instructions which may be included in an instruction block. For the present embodiment, three positions are included. The first position ("position I0") stores the instruction identification information corresponding to the instruction which is foremost in program order within the instruction block stored in the subqueue 86. The second position ("position I1") stores the instruction identification information corresponding to the second instruction in program order within the instruction block. Finally, the third position ("position I2") stores the instruction identification information corresponding to the last instruction in program order. Alternatively, position I2 may store instruction identification information corresponding to an overflow instruction. Certain instruction identification information is the same for each instruction (e.g. the segment limit). To avoid duplicating information, this instruction information may be stored as a single copy separate from the instructions positions.

Control unit 90 maintains the information stored in each subqueue 86. In particular, control unit 90 directs each subqueue 86 to shift instruction identification information between the positions when instructions are selected for dispatch. For example, if the instruction corresponding to position I0 is dispatched, the information stored in position I1 is shifted into position I0 and the information stored in position I2 is shifted into position I1. Similarly, if the instructions corresponding to positions I0 and II are dispatched, then information stored in position I2 is shifted into position I0. In this manner, the instruction within the subqueue which is foremost in program order is maintained in position I0, the instruction which is second in program order is maintained in position I1, etc. In order to select an instruction for dispatch to decode unit 20A, selection control unit 76 examines the instruction identification information stored in position I0 of each subqueue. Advantageously, a small amount of logic may be employed to select the instruction. Similarly, position I0 of subqueue 86A and position I2 of each subqueue 86A-86C are not examined to select an instruction for decode unit 20B. The second instruction to be dispatched will be found within the first two positions of one of the subqueues 66 when maintained in accordance with the above. Selection control unit 76 informs control unit 90 of which instructions positions were selected for dispatch during a clock cycle, such that subqueue shifting may be performed.

According to one embodiment, instruction identification information is shifted internally to each subqueue 86 independently. Instruction identification information is not, therefore, shifted from position I0 of subqueue 86B into positions within subqueue 86A. Instead, when each of the instructions within subqueue 86A have been dispatched, subqueue 86B is shifted into subqueue 86A as a whole. The logic for shifting between subqueues 86 may operate independently from and in parallel with the internal shifting of each subqueue 86A-86C.

Position I2 may store instruction identification information regarding an overflow instruction. If position I2 is storing information regarding an overflow instruction, then the information is not shifted to position I0 or I1 as described above. In this manner, overflow instruction information is always available in position I2. Selection control unit 76 may examine the information stored in position I2 for routing bytes corresponding to an overflow instruction, as opposed to having to locate the overflow information within the positions and then determining byte routing.

Selection control unit 76 selects instructions from the instruction positions within subqueues 86 for potential dispatch. The instructions selected are the instructions which are foremost in program order among the instructions stored in subqueues 86. The operation of select control unit 76 with respect to double dispatch instructions is discussed next.

More instructions are initially selected for dispatch than the number of issue positions included in microprocessor 10, in order to correctly perform redispatch of instructions when an MROM instruction and a fast path instruction are concurrently dispatched and the MROM instruction is found to be an arbitrary dispatch instruction. Selection control unit 76 then selects from the potentially dispatchable instructions based upon the value of a packed state stored in a packed state register 92 coupled to selection control unit 76 and the state of the double dispatch signal upon double dispatch conductor 68, also coupled to selection control unit 76.

When selection control unit 76 selects an MROM instruction and a fast path instruction for concurrent dispatch during a clock cycle, selection control unit 76 sets the packed state. Otherwise, the packed state is reset. The packed state so generated is stored into packed state register 92 for use during the succeeding clock cycle. Additionally, selection control unit 76 informs control unit 90 that the first of the MROM instruction and the fast path instruction (in program order) is being dispatched. In this manner, byte queue 54 retains the second of the two instructions in program order, despite the dispatch of the second of the two instructions. In one embodiment, the packed state comprises a bit indicative, when set, that an MROM instruction and a fast path instruction were concurrently dispatched in the previous clock cycle.

From the potentially dispatchable instructions, selection control unit 76 selects instructions for dispatch based upon the packed state stored in packed state register 92 and the double dispatch signal. If the packed state is set, an MROM instruction and a fast path instruction were concurrently dispatched in the previous clock cycle. Therefore, the instruction within the potentially dispatchable instructions which is foremost in program order is one of the two instructions previously dispatched when the packed state is set. If the packed state is set and the double dispatch signal is asserted, the concurrent dispatch of the MROM instruction and the fast path instruction is successful. If the packed state is set and the double dispatch signal is deasserted, the concurrent dispatch of the MROM instruction and the fast path instruction is unsuccessful. The MROM instruction occupies at least three issue positions, and therefore the fast path instruction cannot be concurrently dispatched for the embodiment of microprocessor 10 shown in FIG. 1. If the packed state is clear, concurrent dispatch of an MROM and fast path instructions was not performed in the previous clock cycle. Therefore, the instructions within the potentially dispatchable instructions were not previously dispatched.

According to one embodiment, selection control unit 76 selects the foremost instructions in program order from the set of potentially dispatchable instructions if either the packed state is clear or the packed state is set and the double dispatch signal is deasserted. In the case of the packed state being clear, the foremost set of instructions are dispatched and program order is maintained. In the case of the packed state being set and the double dispatch signal being deasserted, the second of the instructions dispatched during the previous clock cycle is redispatched. If the second of the instructions is the MROM instruction, it is dispatched alone. If the second of the instructions is the fast path instruction, additional instructions may be selected for concurrent dispatch. Advantageously, the largest number of concurrently dispatchable instructions is selected, even in the case of redispatching a previously dispatched instruction.

If the packed state is set and the double dispatch signal is asserted, then the instruction within the potentially dispatched instructions which is foremost in program order is the second of the previously dispatched instructions and that instruction is successfully dispatched during the previous clock cycle (i.e. the MROM instruction and fast path instruction, when taken together, occupy a number of issue positions less than or equal to the number of issue positions included within microprocessor 10). This instruction is therefore not selected during the current clock cycle. Instead, instructions are dispatched from the remaining of the potentially dispatchable instructions.

Upon selection of the instructions dispatched, the packed state is determined for the subsequent clock cycle. In addition, control unit 90 is informed of the instructions dispatched. For the case of the packed state being set and the double dispatch signal being asserted, the instruction which was previously dispatched is indicated as dispatched as well as each of the instructions dispatched during the present clock cycle. Subqueues 86 are shifted accordingly. In one embodiment, control unit 90 is informed of the subqueue and position storing the last instruction (in program order) to be dispatched. Selection control unit 76 identifies the last instruction in accordance with the above functionality. Byte queue 74 shifts out the instructions prior to and including the indicated last instruction. In this manner, byte queue 74 operates independent of the logic used to concurrently dispatch MROM and fast path instructions. For example, when packing an MROM instruction and a fast path instruction, the first of the instructions in program order is marked as the last instruction. The second of the instructions is thereby retained in byte queue 74 while the first of the instructions is shifted out.

The operation of selection control unit 76 with respect to the early exit signal from MROM unit 34 is discussed next. Selection control unit 76 receives the early exit signal from sequence control 65 (FIG. 3) via early exit signal line 70. The early exit signal indicates whether the next microcode line issued by MROM access 64 is the last microcode line in a microcode sequence that implements an MROM instruction and the number of microcode instructions in the last microcode line. If the number of microcode instructions is less than the number of issue positions, selection control 76 may pack fastpath instructions after the microcode instructions. For example, in the embodiment illustrated in FIG. 2, three issue positions are available. If the last microcode line of an MROM instruction microcode sequence contains only two microcode instructions, selection control unit 76 may pack one fastpath instruction after the microcode line. Likewise, if the pack signal indicates that the last microcode line contains one microcode instruction, selection control unit 76 may pack two fastpath instructions after the microcode line. By packing fastpath instructions at the end of microcode lines, each issue position is utilized.

Selection control unit 76 packs the fastpath instructions following the microcode instructions. Because the fastpath instructions are later in program order than the MROM instruction, the fastpath instructions are packed subsequent to the microcode instructions. For example, if the last microcode line includes two microcode instructions, the two microcode instructions are dispatched to decode unit 20A and decode unit 20B. The fastpath instruction foremost in program order is dispatched to decode unit 20C. In addition, control unit 90 is informed of the number of fastpath instructions dispatched, and subqueues 86 are shifted accordingly. The packed fastpath instructions are dispatched from byte queue 74 to the proper issue position by multiplex to issue 78.

It is noted that additional details regarding the operation of byte queue 74 may be found in the commonly assigned, co-pending patent application entitled: "A Byte Queue Divided into Multiple Subqueues for Optimizing Instruction Selection Logic", filed concurrently herewith by Narayan, et al. The disclosure of the referenced patent application is incorporated herein by reference in its entirety. It is further noted that additional details regarding the operation of double-dispatch instructions and the sequence control unit may be found in the commonly assigned, co-pending application entitled: "A Method For Concurrently Dispatching Microcode And Directly-Decoded Instructions In A Microprocessor", filed on Jul. 24, 1996 by Narayan et al., Ser. No. 08/685,656. The disclosure of the referenced patent application is incorporated herein by reference in its entirety.

Turning now to FIG. 9, a computer system 200 including microprocessor 10 is shown. Computer system 200 further includes a bus bridge 202, a main memory 204, and a plurality of input/output (I/O) devices 206A-206N. Plurality of I/O devices 206A-206N will be collectively referred to as I/O devices 206. Microprocessor 10, bus bridge 202, and main memory 204 are coupled to a system bus 208. I/O devices 206 are coupled to an I/O bus 210 for communication with bus bridge 202.

Bus bridge 202 is provided to assist in communications between I/O devices 206 and devices coupled to system bus 208. I/O devices 206 typically require longer bus clock cycles than microprocessor 10 and other devices coupled to system bus 208. Therefore, bus bridge 202 provides a buffer between system bus 208 and input/output bus 210. Additionally, bus bridge 202 translates transactions from one bus protocol to another. In one embodiment, input/output bus 210 is an Enhanced Industry Standard Architecture (EISA) bus and bus bridge 202 translates from the system bus protocol to the EISA bus protocol. In another embodiment, input/output bus 210 is a Peripheral Component Interconnect (PCI) bus and bus bridge 202 translates from the system bus protocol to the PCI bus protocol. It is noted that many variations of system bus protocols exist. Microprocessor 10 may employ any suitable system bus protocol.

I/O devices 206 provide an interface between computer system 200 and other devices external to the computer system. Exemplary I/O devices include a modem, a serial or parallel port, a sound card, etc. I/O devices 206 may also be referred to as peripheral devices. Main memory 204 stores data and instructions for use by microprocessor 10. In one embodiment, main memory 204 includes at least one Dynamic Random Access Memory (DRAM) and a DRAM memory controller.

It is noted that although computer system 200 as shown in FIG. 9 includes one bus bridge 202, other embodiments of computer system 200 may include multiple bus bridges 202 for translating to multiple dissimilar or similar I/O bus protocols. Still further, a cache memory for enhancing the performance of computer system 200 by storing instructions and data referenced by microprocessor 10 in a faster memory storage may be included. The cache memory may be inserted between microprocessor 10 and system bus 208, or may reside on system bus 208 in a "lookaside" configuration.

Although the x86 microprocessor architecture and instruction set have been used as a specific example herein, it is noted that the apparatus and method described herein may be applicable to any microprocessor which employs microcode and directly-decoded instructions. Such embodiments are contemplated.

It is still further noted that the present discussion may refer to the assertion of various signals. As used herein, a signal is "asserted" if it conveys a value indicative of a particular condition. Conversely, a signal is "deasserted" if it conveys a value indicative of a lack of a particular condition. A signal may be defined to be asserted when it conveys a logical zero value or, conversely, when it conveys a logical one value. Additionally, various values have been described as being discarded in the above discussion. A value may be discarded in a number of manners, but generally involves modifying the value such that it is ignored by logic circuitry which receives the value. For example, if the value comprises a bit, the logic state of the value may be inverted to discard the value. If the value is an n-bit value, one of the n-bit encodings may indicate that the value is invalid. Setting the value to the invalid encoding causes the value to be discarded. Additionally, an n-bit value may include a valid bit indicative, when set, that the n-bit value is valid. Resetting the valid bit may comprise discarding the value. Other methods of discarding a value may be used as well.

Table 1 below indicates fast path, double dispatch, and MROM instructions for one embodiment of microprocessor 10 employing the x86 instruction set:

                  TABLE 1                                                          ______________________________________                                         x86 Fast Path, Double Dispatch, and MROM Instructions                          X86 Instruction    Instruction Category                                        ______________________________________                                         AAA                MROM                                                        AAD                MROM                                                        AAM                MROM                                                        AAS                MROM                                                        ADC                fast path                                                   ADD                fast path                                                   AND                fast path                                                   ARPL               MROM                                                        BOUND              MROM                                                        BSF                fast path                                                   BSR                fast path                                                   BSWAP              MROM                                                        BT                 fast path                                                   BTC                fast path                                                   BTR                fast path                                                   BTS                fast path                                                   CALL               fast path/double dispatch                                   CBW                fast path                                                   CWDE               fast path                                                   CLC                fast path                                                   CLD                fast path                                                   CLI                MROM                                                        CLTS               MROM                                                        CMC                fast path                                                   CMP                fast path                                                   CMPS               MROM                                                        CMPSB              MROM                                                        CMPSW              MROM                                                        CMPSD              MROM                                                        CMPXCHG            MROM                                                        CMPXCHG8B          MROM                                                        CPUID              MROM                                                        CWD                MROM                                                        CWQ                MROM                                                        DDA                MROM                                                        DAS                MROM                                                        DEC                fast path                                                   DIV                MROM                                                        ENTER              MROM                                                        HLT                MROM                                                        IDIV               MROM                                                        IMUL               double dispatch                                             IN                 MROM                                                        INC                fast path                                                   INS                MROM                                                        INSB               MROM                                                        INSW               MROM                                                        INSD               MROM                                                        INT                MROM                                                        INTO               MROM                                                        INVD               MROM                                                        INVLPG             MROM                                                        IRET               MROM                                                        IRETD              MROM                                                        Jcc                fast path                                                   JCXZ               double dispatch                                             JECXZ              double dispatch                                             JMP                fast path                                                   LAHF               fast path                                                   LAR                MROM                                                        LDS                MROM                                                        LES                MROM                                                        LFS                MROM                                                        LGS                MROM                                                        LSS                MROM                                                        LEA                fast path                                                   LEAVE              double dispatch                                             LGDT               MROM                                                        LIDT               MROM                                                        LLDT               MROM                                                        LMSW               MROM                                                        LODS               MROM                                                        LODSB              MROM                                                        LODSW              MROM                                                        LODSD              MROM                                                        LOOP               double dispatch                                             LOOPcond           MROM                                                        LSL                MROM                                                        LTR                MROM                                                        MOV                fast path                                                   MOVCC              fast path                                                   MOV.CR             MROM                                                        MOV.DR             MROM                                                        MOVS               MROM                                                        MOVSB              MROM                                                        MOVSW              MROM                                                        MOVSD              MROM                                                        MOVSX              fast path                                                   MOVZX              fast path                                                   MUL                double dispatch                                             NEG                fast path                                                   NOP                fast path                                                   NOT                fast path                                                   OR                 fast path                                                   OUT                MROM                                                        OUTS               MROM                                                        OUTSB              MROM                                                        OUTSW              MROM                                                        OUTSD              MROM                                                        POP                double dispatch                                             POPA               MROM                                                        POPAD              MROM                                                        POPF               MROM                                                        POPFD              MROM                                                        PUSH               fast path/double dispatch                                   PUSHA              MROM                                                        PUSHAD             MROM                                                        PUSHF              fast path                                                   PUSHFD             fast path                                                   RCL                MROM                                                        RCR                MROM                                                        ROL                fast path                                                   ROR                fast path                                                   RDMSR              MROM                                                        REP                MROM                                                        REPE               MROM                                                        REPZ               MROM                                                        REPNE              MROM                                                        REPNZ              MROM                                                        RET                double dispatch                                             RSM                MROM                                                        SAHF               fast path                                                   SAL                fast path                                                   SAR                fast path                                                   SHL                fast path                                                   SHR                fast path                                                   SBB                fast path                                                   SCAS               double dispatch                                             SCASB              MROM                                                        SCASW              MROM                                                        SCASD              MROM                                                        SETcc              fast path                                                   SGDT               MROM                                                        SIDT               MROM                                                        SHLD               MROM                                                        SHRD               MROM                                                        SLDT               MROM                                                        SMSW               MROM                                                        STC                fast path                                                   STD                fast path                                                   STI                MROM                                                        STOS               MROM                                                        STOSB              MROM                                                        STOSW              MROM                                                        STOSD              MROM                                                        STR                MROM                                                        SUB                fast path                                                   TEST               fast path                                                   VERR               MROM                                                        VERW               MROM                                                        WBINVD             MROM                                                        WRMSR              MROM                                                        XADD               MROM                                                        XCHG               MROM                                                        XLAT               fast path                                                   XLATB              fast path                                                   XOR                fast path                                                   ______________________________________                                    

Note: Instructions including an SIB byte are also considered double dispatch instructions.

It is noted that a superscalar microprocessor in accordance with the foregoing may further employ the latching structures as disclosed within the co-pending, commonly assigned patent application entitled "Conditional Latching Mechanism and Pipelined Microprocessor Employing the Same", Ser. No. 08/400,608 filed Mar. 8, 1995, by Pflum et al. The disclosure of this patent application is incorporated herein by reference in its entirety.

It is further noted that aspects regarding array circuitry may be found in the co-pending, commonly assigned patent application entitled "High Performance Ram Array Circuit Employing Self-Time Clock Generator for Enabling Array Access", Ser. No. 08/473,103 filed Jun. 7, 1995 by Tran. The disclosure of this patent application is incorporated herein by reference in its entirety.

It is additionally noted that other aspects regarding superscalar microprocessors may be found in the following co-pending, commonly assigned patent applications: "Linearly Addressable Microprocessor Cache", Ser. No. 08/146,381, filed Oct 29, 1993 by Witt; "Superscalar Microprocessor Including a High Performance Instruction Alignment Unit", Ser. No. 08/377,843, filed Jan. 25, 1995 by Witt, et al; "A Way Prediction Structure", Ser. No. 08/522,181, filed Aug. 31, 1995 by Roberts, et al; "A Data Cache Capable of Performing Store Accesses in a Single Clock Cycle", Ser. No. 08/521,627, filed Aug. 31, 1995 by Witt, et al; "A Parallel and Scalable Instruction Scanning Unit", Ser. No. 08/475,400, filed Jun. 7, 1995 by Narayan; and "An Apparatus and Method for Aligning Variable-Byte Length Instructions to a Plurality of Issue Positions", Ser. No. 08/582,473, filed Jan. 2, 1996 by Narayan, et al. The disclosure of these patent applications are incorporated herein by reference in their entirety.

Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications. 

What is claimed is:
 1. A microcode instruction unit comprising:a storage device configured to store a plurality of microcode instructions, wherein a subset of said plurality of microcode instructions effectuates a function of a complex instruction, and said complex instruction includes an opcode byte and a prefix byte; a plurality of address generators configured to generate an address of a first microcode instruction of said plurality of microcode instructions that effectuate said function of said complex instruction, wherein a first address generator generates a first address based on said opcode byte and a first presumed prefix byte, and a second address generator generates a second address based on said opcode byte and a second presumed prefix byte; and a selection circuit coupled to said plurality of address generators and to said storage device, wherein said selection circuit is configured to select said first address if said first presumed prefix byte corresponds to said prefix byte of said complex instruction, and said selection circuit is configured to select said second address if said second presumed prefix byte corresponds to said prefix byte of said complex instruction.
 2. The microcode instruction unit of claim 1 wherein said selection circuit comprises:a decoder configured to provide an output indicative of a state of said prefix byte of said complex instruction; and a multiplexor coupled to said decoder, wherein said multiplex is configured to select either said first address or said second address based on said output of said decoder.
 3. The microcode instruction unit of claim 2 wherein said decoder is configured to provide said output in parallel with said plurality of address generators generating said first and second addresses.
 4. The microcode instruction unit of claim 1 wherein said plurality of address generators are programmable logic arrays.
 5. The microcode instruction unit of claim 1 wherein said plurality of address generators generates said first and second addresses further based on an addressing mode byte.
 6. A method of accessing a microcode sequence that effectuates a function of a complex instruction comprising the steps of:generating a first address based on a first presumed prefix byte; generating a second address based on a second presumed prefix byte; selecting an entry point address from either said first address or said second address based on a state of a prefix byte of said complex instruction; and accessing a first microcode instruction of said microcode sequence using said entry point address.
 7. The method of accessing a microcode sequence of claim 6 further comprising the step of decoding said prefix byte to determine said state of said prefix byte.
 8. The method of accessing a microcode sequence of claim 7 wherein said step of decoding said prefix byte is performed in parallel with said steps of generating said first and second addresses.
 9. The method of accessing a microcode sequence of claim 6 wherein said generation of said first and second addresses is further based on an opcode byte of said complex instruction.
 10. The method of accessing a microcode sequence of claim 9 wherein said generation of said first and second addresses is further based on an addressing mode byte of said complex instruction.
 11. The method of accessing a microcode sequence of claim 6 wherein said steps of generating a first and second address is performed by programmable logic arrays.
 12. A microprocessor comprising:an instruction cache; a first instruction unit coupled to said instruction cache; a microcode instruction unit coupled to said instruction cache including:a storage device configured to store a plurality of microcode instructions, wherein a subset of said plurality of microcode instructions effectuates a function of a complex instruction, and said complex instruction includes an opcode byte and a prefix byte; a plurality of address generators configured to generate an address of a first microcode instruction of said plurality of microcode instructions that effectuate said function of said complex instruction, wherein a first address generator generates a first address based on said opcode byte and a first presumed prefix byte, and a second address generator generates a second address based on said opcode byte and a second presumed prefix byte; and a selection circuit coupled to said plurality of address generators and to said storage device, wherein said selection circuit is configured to select said first address if said first presumed prefix byte corresponds to said prefix byte of said complex instruction, and said selection circuit is configured to select said second address if said second presumed prefix byte corresponds to said prefix byte of said complex instruction; a decode unit coupled to said first instruction unit and said microcode instruction unit; and a functional unit coupled to said decode unit.
 13. The microcode instruction unit of claim 12 wherein said selection circuit comprises:a decoder configured to provide an output indicative of a state of said prefix byte of said complex instruction; and a multiplexor coupled to said decoder, wherein said multiplex is configured to select either said first address or said second address based on said output of said decoder.
 14. The microcode instruction unit of claim 13 wherein said decoder is configured to provide said output in parallel with said plurality of address generators generating said first and second addresses.
 15. The microcode instruction unit of claim 12 wherein said plurality of address generators are programmable logic arrays.
 16. The microcode instruction unit of claim 12 wherein said plurality of address generators generates said first and second addresses further based on an addressing mode byte. 